1. Field of the Invention
The present invention relates to a sintered body of silicon nitride suitable for use as a high-temperature structural material owing to its high strength and high toughness.
2. Description of the Prior Art
A sintered body of silicon nitride (Si.sub.3 N.sub.4) has been used as a heat-resistant structural material such as gas turbine parts, heat exchanger material, high-temperature bearings, and high-temperature rolls for steel making, owing to its outstanding high-temperature strength, thermal shock resistance, and corrosion resistance.
A disadvantage of silicon nitride is that it needs a sintering aid, which is usually an oxide such as MgO, MgAl.sub.2 O.sub.4, Al.sub.2 O.sub.3, and Y.sub.2 O.sub.3, because it involves difficulties when it is sintered alone. It is considered that the sintering aid brings about liquid-phase sintering through the medium of a liquid phase which forms during sintering. After sintering, the liquid phase remains as a glass phase in the sintered body, impairing the high-temperature characteristics such as strength and creep resistance.
Silicon nitride can occlude a variety of elements to yield a material generally called sialon. It is regarded as a future high-temperature structural material owing to its high-temperature characteristics. There are two types of sialon: .alpha.-sialon (or .alpha.'-Si.sub.3 N.sub.4) and .beta.-sialon (or .beta.'-Si.sub.3 N.sub.4). .alpha.-sialon has the .alpha.-Si.sub.3 N.sub.4 structure and is represented by the formula M.sub.2 (Si,Al).sub.12 (O,N).sub.16 (where 0&lt;x .ltoreq.2, and M denotes at least one member selected from metallic elements, such as Li, Mg, Ca, and Y). It has Al and O substituted at the Si position and N position, respectively, and it also has other elements (such as Li, Mg, Ca, and Y) penetrated to the interlattice position. .beta.-Sialon has the .beta.-Si.sub.3 N.sub.4 structure and is represented by the formula Si.sub.6-y Al.sub.y O.sub.y N.sub.8-y (where 0&lt;y .ltoreq.4.2). It has Al and 0 substituted at the Si position and N position, respectively.
Unfortunately, sialon composed of .alpha.'-Si.sub.3 N.sub.4 alone or .beta.'-Si.sub.3 N.sub.4 alone is inferior to a sintered body of silicon nitride in strength at room temperature and toughness. To eliminate this disadvantage, there have been developed sintered bodies composed of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 as mentioned below.
It is reported in J. Materials Sci. 14 (1979p. 1749 that it is possible to obtain a sintered body based on a single phase of .alpha.'-Si.sub.3 N.sub.4 or a mixed phase of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 by sintering a mixture of Si.sub.3 N.sub.4, Y.sub.2 O.sub.3, and AlN powders. Also, it is reported in Japanese Patent Laid-open No. 185484/1983 that a sintered body based on a mixed phase of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 is obtained from .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 powders. Unfortunately, these two sintered bodies are still poor in strength at both room temperature and high temperatures.
In addition, it is reported in Japanese Patent Laid-open No. 182276/1984 that a sintered body composed of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 is obtained by sintering a mixture of Si.sub.3 N.sub.4, Y.sub.2 O.sub.3, AlN, and Al.sub.2 O.sub.3 powders. This sintered body is claimed to have an improved strength at high temperatures, if it contains .alpha.'-Si.sub.3 N.sub.4 in a ratio of 0.05-0.7 and it also contains crystal grains having a diameter smaller than 40 .mu.m in the major axis.